This invention relates to an improvement in the known type of oxygen sensor comprising a solid oxygen-ion-conducting electrolyte with porous, thin layer or film, metal (e.g. platinum) electrodes attached on substantially opposite surfaces of the electrolyte. When each electrode of this type of sensor is in contact with a different oxygen concentration and the electrodes are connected in an electrical measuring circuit, oxygen ions migrate through the electrolyte between the electrodes coincidently with a flow of electrons in the circuit generating a measurable voltage or electromotive force between (or across) the electrodes or two points in the circuit.
This type of sensor has been known for use in monitoring: (1) exhaust gases of internal combustion engines in thermodynamic nonequilibrium for control of the air-fuel ratio in the combustion process, (2) stack or flue gases of industrial combustion furnaces for control of the combustion process to eliminate smoke and other undesirable emissions, and (3) furnace atmospheres of metal heat treating and other furnaces in substantial thermodynamic equilibrium for control of their oxygen potential, e.g. in nonoxidizing and reducing gas atmospheres.
However, durability of the thin layer or film metal electrode in contact with the monitored flowing hot gases has been a problem. Such electrode has been variously noted to be adversely affected by thermal shock and differential expansion stresses in the sensor, mechanical abrasion and impact stresses caused by particles carried in the flowing gases, and chemical reaction effects with constituents in the gases being monitored.
In U.S. Pat. No. 3,645,875, it is noted that reducing gas atmospheres and metal vapors in such atmospheres of metal heat treating furnaces cause embrittlement of platinum film electrodes and adversely affect the bond between such electrode and the solid electrolyte. The remedy suggested in this patent for such problems is adherently attaching a thin, porous, protective overlayer on the platinum film electrode and on the adjacent electrolyte surface not covered by the electrode so as to secure the electrode on the electrolyte. Such adherent overlayer is applied by firing a paste coating or flame spraying a coating of the overlayer on the electrode and electrolyte surfaces.
A similar remedy is shown in U.S. Pat. Nos. 3,978,006, 4,021,326 and 4,126,532 to protect a catalytic metal film electrode on a sensor against mechanical and chemical damage in internal combustion engine exhaust gases. Additional application techniques noted therein include plasma spraying, metal plating followed by oxidation and various other thin-layer techniques such as thermal vaporizing, precipitation from gases and reactive vapor deposition.
Other examples of thin, porous coatings similarly adherently applied only onto platinum film electrodes of sensors, mostly for internal combustion engines, are shown in U.S. Pat. Nos. 3,935,089, 4,080,276, 4,097,353 and 4,164,462, and in Japanese laid-open patent application publication No. 54-10792. However, U.S. Pat. No. 4,164,462 notes that some of such adherent, porous overlayers or coatings can suffer a durability problem (cracking) of their own. This latter fact was confirmed in my studies, which showed that such overlayers (e.g. of alumina cement) readily crack and spall off, thereby leaving significant portions of the metal film electrode uncovered. A consequence of such results is that embrittled and loosened metal film electrodes can crack and flake off or separate from the electrolyte causing early failure of the sensor.
Trying to improve the strength and adherence of the overlayer by firing it on the sensor at higher temperatures above about 1150.degree. C. is often unsatisfactory, especially for sensors with stabilized zirconia electrolytes to be used in metal heat treating processes. Besides the possibility of destroying the needed porosity by sintering the overlayer too dense, reheating of the stabilized zirconia electrolyte (while firing the overlayer) above about 1150.degree. C. causes a change in the zirconia structure, which in turn causes the electrolyte to exhibit sluggish, nonideal behavior in service at temperatures below about 1150.degree. C. as is often the case in metal heat treating processes.
Another effort to overcome the metal film electrode adherence problem in sensors for automotive exhaust gas is shown in Japanese laid-open patent application publication No. 53-29188. This effort involved leaving holes in the electrode with exposed electrolyte surface being covered, along with the electrode, with a porous thin layer of inorganic compound to prevent stripping and scattering of the electrode.
Although not concerned with a problem of keeping metal film electrodes adhered to a solid electrolyte surface, U.S. Pat. No. 4,121,989 shows a specially tailored oxygen sensor device for greater efficiency, accuracy and reproducible operation in monitoring stack gases from industrial combustion furnaces. Such device has felted ceramic fiber discs partly embedded into wet electrode paste of coatings and fired therein. Then chloroplatinic acid is applied through the felted discs and thermally reduced to platinum particles dispersed over the electrode surfaces and within the felted discs to augment the capacity of the platinum electrodes to effect the ionization-deionization reactions of oxygen in the device.